1. Field of the Invention
The present invention relates to a novel and useful glass ceramic material, to a process for the manufacture thereof, and to the use of the resultant material, in particular, as bone replacement material.
2. Description of the Prior Art
For osteoplastics and osteosynthesis there are employed, in addition to bone transplants, also metals such as, for instance, silver and tantalum, metal compounds such as, for instance, the surgical alloy steel "Vitallium" or the chromium-cobalt alloy "Protasul 1", and plastics such as, for instance, polyethylene, methacrylates, or silicone rubber.
While this tolerance of the body for some of the said implants in the animal or human organism can be considered satisfactory, none of the said materials or the materials known or tried out up to the present time are able to grow together with the bone in the living organism.
As is known, the substance of animal or human bone consists essentially of hydroxyapatite (Ca.sub.5 [(OH)(PO.sub.4).sub.3 ]) which is permeated, in an intimate mixture, by albuminoids (collagen). The great difference in chemical composition between the bone-replacement material and the bone itself is the reason why the synthetic bone-replacement materials known up to the present time do not grow together with the mass of the bone.
Therefore, up to now it has been only possible, by suitable shaping of the implant, to obtain a certain mechanical anchoring, as a result whereof the tissue close to the bone simply envelopes the replacement material. The contact thus produced between the artificial implant and the bone, however, always remains weak and in particular cannot be subjected to the usual forces or stresses.
It is already known that regeneration of bone substances proceeds from the mineral hydroxyapatite. This substance evidently acts here as point of attachment for the albuminoids of the bone substance. Starting from hydroxyapatite nuclei, a complete bone is thus regenerated and built up. At the same time, connections to and with the bone fragments which are still present are also formed, i.e. formation of callus takes place.
Now, in itself, it would be possible to use apatite in sintered form as bone-replacement material. This method would, however, have the definite disadvantage that in order to obtain sufficient initial stability of the prosthesis or prosthesis part, there would have to be introduced very large quantities of apatite which exceed by far the quantities required for the synthesis of a bone. Under such conditions, regeneration and final development of a bone replacement capable of bearing loads, however, will require too much time.
L. L. Hench, R. J. Splinter, T. K. Greenlee, and W. C. Allen have proposed, in an article entitled "Bonding mechanisms at the interface of ceramic prosthetic materials," the use, as bone replacement, of apatite-containing materials which, as such, possess sufficient strength properties, so that, upon their intergrowth, full load-bearing capacity is obtained immediately. In said article, there are proposed glass ceramic materials in which a sufficient number of apatite nuclei are produced by suitable thermal treatment, so that the growth of the bone can take place in a known manner on such nuclei. Thus it can be assumed with some degree of certainty that such a glass ceramic material will grow together with the existing bone in situ.
The glass ceramic materials proposed by Hench et al., however, have serious disadvantages which can cause their use in the animal or human oraganism to become a serious source of danger for the animal or person bearing the implant, especially when implanting large replacement pieces.
Since it is known that the ratio of the two ions Na.sup.+ and K.sup.+ to each other is a decisive factor for proper functioning of the nerves and muscles in the animal organism, even relatively small variations and displacements, in particular of the potassium ion concentration will change the excitability or responsiveness of the nerves and thus will lead to serious impairment of the heart. Said ratio of sodium to potassium ions is all the more important because the extracellular potassium ion concentration, which in general is the only important factor, constitutes only about 2 percent of the total potassium content of an organism. Disturbances in this small amount of extracellular material which, as a whole, constitutes only about 2 g. to 3 g. of potassium ions can be caused by even only relatively small shifts in the potassium content of the blood or lymph.
Similar considerations must also be made with respect to the Mg.sup.2.sup.+ and Ca.sup.2 .sup.+ ions which are also present in the animal and human organism in a substantially invariable ratio and in a concentration which is also invariable. Changes and shifting of said ratio necessarily result in severe damage in the organism in question.
The glass ceramics proposed by Hench et al. are prepared from pure sodium-calcium glasses.
It is self-evident that, in view of the known capability of glass to act as ion exchanger, there exists a potential of sodium Na.sup.+ ions and calcium Ca.sup.2.sup.+ ions which, one the one hand, will greatly change, by leaching out, the concentration of these two ions at the area surrounding the glass ceramic implant and which, on the other hand, will also considerably reduce by exchange adsorption the concentration of the ion antagonists, for instance, of potassium K.sup.+ ions and magnesium Mg.sup.2.sup.+ ions. Thus it is to be expected that, when implanting larger replacement pieces, the effect on the specific ion concentration will extend, as a function of the geometric shape of the implant, to more remote organs and their functions. As a result thereof a high displacement of the ionic ratios is to be expected especially when the ceramics are used in the form of porous sintered or foam materials, i.e. in a form which is particularly flavorable for technical-medical reasons.
Another substantial disadvantage of the known glass ceramics is their relatively low tendency to form nuclei. This leads to extremely long and technically expensive recrystallization processes. Furthermore, the number of nuclei formed per unit of volume is very difficult to control technologically since it is dependent on numerous imponderable factors such as the degree of purity of the chemical starting materials, the prior heat treatment or thermal antecedents of the glass, the material of which the crucible consists, the constancy of the heating program, and others.